The invention relates to the field of marine geo-electrical survey, in particular, to surveys utilizing methods of induced polarization, and is intended for providing prognosis on hydrocarbon deposits in sea-bottom strata.
Nowadays there are known numerous widely used methods for marine survey of hydrocarbon deposits, typically including: a probing (sounding) of the sea-bottom by pulses of electromagnetic field, registration of changes of electromagnetic parameters of the sea near-bottom strata, preliminary processing and analysis of the obtained data for detection of existing anomalies of the strata, and determination of the nature of such anomalies (e.g., RU 2236028, 2004; SU 1122998, 1984; SU 1798666, 1996; SU 1434385, 1988; U.S. Pat. No. 4,298,840, 1981; U.S. Pat. No. 4,617,518, 1986, etc.), whereat employing different research complexes of equipment (herein further called ‘RC’).
Exemplarily, a method for marine electric survey and a research complex used therefor are known, having a conventional name CSEM (L. MacGregor, M. Sinha /Geophysical Prospecting, 2000, 48, 1091-1106; UK. Pat. No. 2402745, issued 2003), allowing to execute the survey at sea depths up to 3 km. The essence of this method is that electromagnetic pulses are transmitted from a horizontal dipole with a momentum of about 104 Am; the dipole is towed by a vessel in a region, wherein sea-bottom stations are installed at a distance about 50 m from the sea-bottom, and the sea-bottom stations radiate a continuous pulse signal of electromagnetic field with a frequency in the range of 0.25-4 Hz. The signal is affected by the environment, and is further received by the sea-bottom stations. Since the resistivity of the sea water is lower than one of the sea-bottom, the signal quickly damps in the water, and, as a result, in case of measuring at the distance more than 500 m from the radiation source, the sea-bottom station receives only the signals related to the sea-bottom strata resistivity. Thusly, the receivers of the sea-bottom stations register two orthogonal components of the horizontal electric field at the distance up to 15 km from the source. Studies of the changes in amplitude and phase of the obtained signal allows receiving information on the electric resistivity of the strata up to depths of 5-7 km. The obtained data are compared with analogous data obtained from a similar region, where the hydrocarbons deposits are absent, and on the base of comparison, a conclusion is drawn on the region perspectives concerning the hydrocarbons deposits.
However, the CSEM technology doesn't allow obtaining data on the strata chargeability that significantly decreases the accuracy of prognosis. Another disadvantage of this method is a relatively low spatial resolution and technological difficulties in the case of using this method at shallow water.
There is known a technology of marine electric survey for oil-and-gas deposits in shelf zones, earlier developed by the instant authors (International Patent Application PCT/EA/2006/000009 hereby entirely incorporated by reference). The essence of the technology is that before the submersion, one synchronizes the clocks mounted on an excitation field formation block in the dipole and on the sea-bottom stations; the dipole is towed by a vessel on the sea surface along an observation profile; the field excitation is carried out by periodical electromagnetic pulses of different polarity with pauses between them; the electrical field registration is carried out by multi-channel sea-bottom stations furnished with braided receiving lines, having at least three electrodes, located at a distance 50-500 meters from one another and allowing to measure both the field potentials and its spatial derivatives along the observation profile; one registers time series of received signals, recording the potentials differences and the spatial derivatives of the electric field, both at on-time and at off-time of current; analyzing the signals, one takes into consideration the change of primary and secondary fields in a time domain and in a space domain simultaneously; and one determines both the strata resistivity and its polarization characteristics. However this technology is not usable for large depths.
There is another technology of sea-bottom research, earlier developed by the instant authors (RU0048645, issued in 2005, hereby entirely incorporated by reference), considered close to the claimed method. According to it, the electric field excitation is carried out by a vertical or horizontal dipole, in which bipolar pulses are formed with pauses therebetween, and the measuring of orthogonal components of the field are executed by the sea-bottom stations. Thus, the basic data are obtained in the time domain, and their inversion allows not only to obtain data on the bottom strata resistivity, but also to mark out chargeability anomalies, related to hydrocarbon deposits.
The most similar related art method to the claimed method is a technology of sea-bottom research, earlier developed by the instant authors (Patent of Russian Federation RU2324956, U.S. Pat. No. 7,529,627, hereby entirely incorporated by reference), according to which: one compares the clocks mounted on a dipole and on sea-bottom stations before the submersion of the sea-bottom stations in a research zone; a vessel tows the vertical dipole placed in the zone of the stations location, so that its upper end would be at a distance not exceeding 200 meters from the sea surface, and its lower end would be at a distance not exceeding 100 meters from the sea-bottom; the electric field excitation is executed by periodical pulses of opposite polarity with pauses between them fixed in time; the obtained information on electric resistivity of the sea-bottom strata from the sea-bottom stations is presented in the form of time series of horizontal and vertical field components, both at on-time and at off-time of current; and in the signals analysis one takes into consideration the change of both the primary and secondary fields in time. Besides the data characterizing the strata resistivity, one determines its polarization characteristics. Based on the resistivity and polarization characteristics, a modeling of the profile of sea-bottom strata is carried out, and a prognosis on hydrocarbon deposits presence is made based on the modeling. This technology has a high geological efficiency; however the available depth of research sharply decreases in the case of presence of polarizing strata within the sea-bottom strata.